Background Information: Sea Surface and Sea Level
It is important not to confuse the shape of the sea surface with the level of the sea surface. Ocean bathymetry (and its effects on the geoid) changes significantly only on time scales of 1 to 10 Ma (1 Ma = one million years), whereas sea level fluctuations occur on time scales of 1 to 10 thousands years. For many purposes, therefore, we can assume the shape of the sea surface (the geoid) to be effectively constant.
The term, geoid is the equipotential surface that would be assumed by the sea surface in the absence of tides, water density variations, currents and atmospheric effects. It may vary above and below the geometrical ellipsoid of revolution by as much as 100 meters due to the uneven distribution of mass within the Earth. The mean sea level varies about the geoid typically by decimeters, but in some cases by more than a meter.
One must have in mind however that the equilibrium balance depends upon equating pressures of adjacent water columns, and this in turn depends on water properties, notably temperature and salinity. For example, a column of a few hundred meters of cold saline water will need to be balanced by a much greater volume of warm low salinity water, so that the latter invariably stands higher providing hills in the water surface. Such topography can create gradients in the ocean surface through a meter or so.
Mean sea level at a location changes in time due to many factors: tides, barometric pressure, wind stress, temperature, salinity, etc.
For practical purposes it is generally taken to be the elevation of the sea surface above a fixed local datum, tidal oscillations having been removed. The tidal contribution can be identified and removed by predictions or by the application of a numerical filter to long series of observations of sea level. Often a quick estimate is used based simply upon the arithmetic mean of hourly observations of sea level over as long a period as possible, often over 19 years, since there does exist a tide which cycles in such a long period.
It is sometimes supposed that the mean sea level determined by averaging the effects of tides and surges over a year or even several years, is the local level of the geoid. Although mean sea level is a good first approximation to the geoid, there are other oceanographic effects such as water density variations, permanent ocean circulation patterns, and atmospheric effects such as mean air pressure and winds, which sustain some semi-permanent displacements of the mean sea level from the geoid.
Changes in Sea Level
When we talk about the sea level changes, we need to specify which kind of sea level change we are focusing upon, since there are different terms based upon the particular study. On the one hand, under the vertical land movements, the main interests include isostatic adjustment, tectonic effects, sedimentation and human factors such as usage of ground water, deforestation and oil extraction. On the other' hand, under the changes in the level of the ocean surface, melting glaciers and ice sheets, ocean currents, daily tides, expansion or contraction of water based upon temperature change and changes in the hydrological cycles are the major interests.
Before we go any further, the definitions of a few useful terms to describe sea level changes may be necessary for a better understanding of the subject. Some of them are quite abstract and even scientists may find it confusing on occasion. From time to time, we have been approached by scientists and the like to clarify some of the definitions. Genuine effort is used to make them as simple as possible. However, readers may need to refer to more advanced text books.
Secular Change: Long-term changes of mean sea level are called secular changes. They are changes of mean sea level at a site over long period of time, typically decades. It is non-periodic tendency of sea level to rise, fall, or remain stationary with time. Typically a trend, [technically, secular change] is frequently defined as the slope of a least-squares line of regression through a relatively long series of yearly mean sea level values.
Eustatic Change: Global changes in the mean sea level, representing changes in the volume of the ocean are named eustatic changes (changes are due to glacial melting or formation, thermal expansion or contraction of seawater, etc.).
Isostatic Adjustment: Where sea level has changed through the addition of mass as in the eustatic case. It is common for the sea-bed to adjust to the change in load, such adjustments are termed isostatic adjustment [readers are requested to try the Activity 4 at the end of this module, this interesting- phenomena will be better understood]. In theory, it is a condition of balance for all large portions of the Earth's crust. For example, as a result of erosion or deposition, this balance is put out of equilibrium and has to be compensated for by movements of the earth's crust. In general, areas of deposition sink, whereas areas of erosion rise.
Epeirogenic Movement: Vertical land movements of regional extent are called epeirogenic movements (direct meaning is making of continents). To be exact, it is an action of uplift or subsidence of large area of continent or ocean basins. For example, sea level can change because of the movement of sea-bed.
One of the major problems of mean sea level interpretation is the identification of separate eustatic changes and isostatic adjustment (and epeirogenic changes) when only secular changes are directly measurable at a particular location.
Causes of Sea Level Change
- Volume of water in the oceans.
- Inputs from: rainfall, snowfall, rivers, ground water, melting ice, volcanism.
- Outputs through evaporation and freezing.
- Properties: Temperature of water, amount of dissolved and suspended matter in it.
- Shape of the basin.
- Global thickness and area of continental crust.
- Relative thermal states of the continental and oceanic crusts (i.e., density and volume of active spreading ridges).
- Mass of water and sediments in the ocean and the resultant load on the ocean crust.
Sea Level and Global Warming
As stated earlier, global temperature is believed to be gradually rising. Because of that, the earth's surface is maintained at a higher temperature than that appropriate to balance the incident solar radiation. Solar radiation penetrates the atmosphere, but some reflected radiation which has the longer-wavelength is absorbed by the carbon dioxide, ozone, water vapor, trace gases and aerosols in the atmosphere. Observed increases in the concentrations of atmospheric carbon dioxide due to burning of fossil fuels, and of other components from industry and other human activity, could lead to a steady increase of global temperature.
The resulting thermal expansion of the oceans and the melting of land ice would increase sea levels. Concern about the possibility of coastal flooding due to this increase has stimulated recent research into climate dynamics. Accordingly, the South Pacific Sea Level and Climate Monitoring Project has been launched by the Australian Government since 1991 and the National Tidal Facility of 77ze Flinders University of South Australia has been designated to implement the project.
We may simply identify the factor that potentially has the greatest effect on the total volume of water in the oceans. The most important factors controlling the volume of water in the oceans is believed to be the balance between the freezing and melting of land ice, such as in mountain glaciers, but also in the thermal expansion of the ocean surface layers.
As mentioned in various basic Physics text books, normal seawater expands in volume by a factor of -3 X 10-4 'C-'. Ignoring gain and loss of water by freezing and thawing, we may expect to find that the effect of I 'C increase in average ocean water temperature of the upper layers over a very long period of time.
A 1 degree Celsius increase in average temperature would cause the volume of the ocean water to increase by a factor of -3 x 10' or 0.03 percent or so. So far as we are aware there is no suggestion that the entire depth of the ocean is likely to be warmed by 1 degree Celsius. Although the average depth of the oceans may be taken as - 3500 meters, only upper 500 meters or so will be considered here as an extreme example. Neglecting any other change in surface area (which may be very small in comparison with the total surface area of the oceans), an average temperature rise of 1 degree Celsius throughout the upper part of oceans (surface to 500 meters) would cause the layer depth to increase by 0.03 percent, due to thermal expansion of the water. This is the rise of:
500 m x - 0. 03 % - 0.15 m - 15 cm
So, ignoring ice sheets and glaciers, global temperature changes will cause a sea level change of the order of 10 centimeters. This does not seem much in relation to the depth of the oceans, but when we are considering changes of the order of millimeters for the low-lying atolls, especially in the Pacific region such as Tuvalu, Marshall Islands, Kiribati and Tokelau, then sea level response to temperature changes could become an important factor.
Seasonal Changes in Mean Sea Level
The monthly variations in mean sea level at a particular location do not necessarily repeat themselves exactly from month to month or year to year. Variations in mean sea level over short periods may be considerably greater but they do not reflect the long term sea level trends. For example, sea level is relatively high in autumn and low in spring months in the respective hemispheres as a result of seasonal water density changes. An example of sea level differences attributable to density differences (although not strictly seasonal in this case) is the -50 centimeters higher sea level off the Pacific coast of the United States compared with the sea level off the Atlantic coast. The Pacific ocean has a lower average density than the Atlantic ocean.
Two seawater characteristics, acting either separately or together, that are capable of causing a change in the density (and volume) of seawater are salinity and temperature. Salinity is important because of its effect on density and volume and therefore on sea level. Temperature is probably more important than salinity because of the expansion or contraction (volume increase or decrease) of water that is caused by temperature increase or decrease. Temperature influences on local and temporal scales are more effective on local and short term. In general, sea level changes related to density are seasonal.
In general, one can infer that sea level is subject to oscillation due to temperature and salinity changes which are in turn based upon seasonal changes in precipitation, evaporation and seasonal wind fields [in particular, Monsoon]. In extreme cases, usually, Monsoonal cases, this annual oscillation can be large in the order of 1 inch. In non-Monsoonal cases, it is much smaller in the order of 10 cm in amplitude.
In addition, effects of atmosphere-ocean interaction, such as El Niño Southern Oscillation [ENSO] can produce large scale variations in mean sea level of up to -50 centimeters with corresponding changes in rate and direction of tidal streams (tidal currents).
Regional Sea Level Change
For regional sea level changes, we refer to the areas of 10 000 k@ or more and from a few tens to millions of years duration. Sea level changes can be due to isostatic adjustment (due to changes in load of ice in the ocean) or epeirogenic changes implying mechanisms at work in the solid Earth, perhaps in the deep earth crust.
In the first case, one might draw attention to regions in the high latitudes in the Northern Hemisphere [in Canada and Scandinavia], where the earth crust is clearly,rising in response to the slow relief of ice-load which was formed during the last ice-age. In Scandinavia, for example, there exists a region where sea level appears to be falling, at the rate of 1 centimeter per year, rather than rising in response to climate change. What is actually happening is that the land has been rising. The increase in elevation of the land is an isostatic response to the removal of the ice load that had depressed the land during Pleistocene.
For the Pacific region in particular, the scale and implications of changing sea level and climate are more complicated since climate is only one part of the problem of changing sea levels. Others include the movement of the earth's crust due to movement of continental plates, active volcanoes, and the earthquakes which all occur in the region as a regular feature. One has in mind, in particular, many islands of Vanuatu are reacting by rising and falling at different rates due to such mechanisms. In these cases, it is necessary to note that the local response of sea level can well be obscured and disturbed by the volcanic and tectonic effects. In order to separate the sea level change due to oceanic processes from those due to crystal processes, the vertical movements of the earth's crust need to be determined.
It is now clear that the vertical land movement is one of the@main factors which can influence the sea level change. Without taking it into account in sea level measurements, sin king and may mislead us as a sea level rise. Accordingly, one of the@main components of the Utah @Pacific Sea Level and Climate Monitoring Project is Surveying and Geo . Regular survey for vertical land movement in the eleven Pacific island countries has been conducted since the project began.
Sometimes, very large scale interaction between ocean and atmosphere can take place over a time scale of the order of, say, 2 to 7 years. The well known El Niño is one such example. Such interactions can be associated with a fairly rapid change of water properties over a large oceanic area. Normal precipitation may be changed, so affecting regional value of the salinity of the ocean and disturbances to ocean currents can help to change the ocean temperature over similar regions. The result is a significant change in regional sea level.
Tides, Sea Level, and Residual
Tides can be clearly noticed as a regular rise and fall of Sea Level at the coast. This regular rise and fall also occurs offshore [generally offshore deep water tides are very small] and in the deep ocean. This applies to most Pacific island countries. This regular rise and fall occurs virtually everywhere throughout the ocean. The largest tides are associated with shallow continental shelves, gulfs and embayments along the continental margin. Tides also produce tidal currents [tidal streams) which are the periodic movement of water from one place to another to accommodate the sea level change. Tides are also known as long gravity waves and shallow water waves and they can be accurately predicted for a particular area from the long continuous record of sea level observations.
Sea level is a measurable physical quantity and is the result of all influences which affect the height of the sea surface (moon, sun, atmospheric pressure, winds, vertical land movement, some oceanographic effects, seismic activity, etc.). Tides are only part of sea level and tide is related in frequency, amplitude and phase to astronomical forcing (i.e., gravitational forces of the moon and the sun on the earth).
Against the back ground of the tidal motion, the determination of long term sea level trends is more difficult. Since clearly the tides may have a range of some meters through which a matter of hours whereas the sea level trend, which is the main focus of the Pacific community, has a magnitude of only 1 or 2 millimeters per year. In order to appreciate sea level trends, it is common practice to take long continuous series of sea level observations, measured with very accurate instruments.
The common terminology, sea level residual is equivalent to "observed sea level minus tide." The residual is that part of the observed change in the sea level which is not due to tides. It is due to meteorological, seismic activity and many other influences, generated locally or perhaps many thousands kilometers away [i.e. Tsunamis]. In sea level data analysis, we usually look at the sea level residual trend or significant residuals (spikes) at a particular time or day to investigate the causes and scientific reasons so as to justify why they occur. Strong winds, high or low atmospheric pressure and water temperature are the usual causes in this respect.
Sea level trends remain contained in the sea level,61 residuals, still obscured by the meteorological or seismic activities, which needs further special treatment, usually by extending the records over several years and so balancing out the short-term disturbances.
General Meteorological Effects on Sea Level
Meteorological conditions, which differ from the average, will cause corresponding differences between the predicted daily tides and the actual height of sea level. Variations from predicted heights are caused mainly by strong or prolonged winds, and by unusually high or low barometric pressure. Difference between predicted and actual times of high and low water are caused mainly by the wind.
Effect of Atmospheric Pressure
The annual tidal predictions booklet and calendar produced by the Sea Level and Climate Monitoring Project since 1996 are computed for average barometric pressure. In fact, a change of barometric pressure by 1 hPa may cause - 1 centimeter variation in sea level. In order to highlight the argument more clearly, it may be necessary to look at the following simple example of atmospheric pressure effect on sea level.
This pressure change is acting upon the sea surface and the following relationship can be used for the variation of sea level. This depression of the water surface under high atmospheric pressure, and its elevation under low atmospheric pressure, is often described as the inverted barometer effect. The water level does not adjust itself immediately to a change of pressure and it responds to the average change in pressure over a considerable area. Changes in sea level due to barometric pressure seldom exceed -30 centimeter, but the effect is important as it is associated with those caused by wind setup since winds are driven by the pressure gradient. During the tropical cyclone period, the inverted barometer effect on sea level change could be quite significant as most Pacific islanders are well aware.
The effect of wind stress (stress is like pressure, force acting on a unit area) on sea level and hence on tidal heights and times is highly variable and it largely depends upon the topography of the land area. The effect of wind stress at the surface of the ocean is transmitted downwards as a result of internal friction within the upper layer of water. For example, the greater the speed of wind, the greater the frictional force acting on the sea surface, and the stronger the surface current will be. The result is a bigger flood at the coastal region if the wind direction is towards the land. From empirical observations, it can be stated that the surface current speed is typically about 2 to 3 percent of the wind speed. This is only a rough rule of thumb. Generally, wind will raise the sea level in the direction of wind speed, which is often called wind setup.
The effect of wind stress is in fact non-linear but increases roughly as the square of the wind speed. For example, if the effect of a wind speed, x ms-I is found to be a sea level perturbation of y meters, then the effect of a wind speed, 2x ms-' is likely to be of the order of 4y meters.
Although it is difficult to give a general rule for the effect, a strong wind blowing onshore will pile up the water near the coast and cause high waters to be higher than predicted. If the wind blows offshore, there will be a reverse effect. In addition, winds blowing along a coast tend to set up long waves which travel along the coast, raising the sea level at the crest and lowering it in the trough.
Effect of Storm Surges
Storm surges are long-period surface waves caused by storm winds, usually tropical cyclones in the Pacific region. In a storm surge, strong winds pile up water along a coast, causing sea level to rise. When a northwest gale, for example, blows across the North Sea with a fetch of 900 kilometers from Scotland to the Netherlands, sea level can rise more than 3 meters. On March 16, 1997, during the period of Tropical Cyclone, Hina, a storm surge, combined with strong waves but fortunately with low tides, hit Tonga and sea level rose about 1 meter.
In scientific term, the combination of wind setup and the inverted barometer effect associated with storms may create a pronounced increase in sea level and is called a storm surge. An additional process in the form of a long surface wave traveling with the storm depression can further exaggerate this sea level increase.
The approximate height of a storm surge can be calculated, based upon the atmospheric pressure change, wind speed and direction, length of fetch [wind affected area], water depth, and shape of the ocean basin. Other factors, such as currents, tides, and seiches set up by storms, complicate the calculations.
A negative surge is the opposite effect, generally associated with high pressure systems and offshore winds, and can create unusually shallow water. This effect is of great importance to very large vessels which may be navigating with small under-keel clearances.
Effect of Local Thermal Expansion
Thermal expansion occurs not only in solids but also in liquids and gases as discussed in the previous section. Unlike solids, gases and liquids do not have well defined shapes and accordingly, the volume expansion needs to be considered. The change of volume, AV, due to temperature change, AT, can be given as follows:
It is interesting to estimate the seasonal change in sea level as a result of solar heating of the upper top layer. Thermal expansion of the upper layer of the ocean at a particular region is generally seasonal and it may not affect the long term sea level change. However, if the global warming is taken into account for a particular locality, that sea level change will be a considerable addition to the regional sea level Accordingly, we may infer that 1 degree Celsius temperature increase over the upper layer of approximately 50 meters will raise the sea level by approximately 1.5 centimeters (some simply say - 1 centimeter).
Miscellaneous Effects
As populations, urbanization and industrialization increase, humanity's water needs grow. An enormous volume of rain and river water is being delayed on its oceanward path by diversion into reservoirs and irrigation projects. This volume, about 375 km3 per year by the mid-1980s, is a significant addition to the total volume of fresh water in the hydrological cycle. Due to this water storage in reservoirs and irrigation projects, there may be suppression of global sea level rise.
Although for a long time the solar radiant energy received by the atmosphere was thought to be a constant, it is now known that there are regular fluctuations, which may be as great as 2 percent or so. Furthermore, the kind of solar radiation may be a more important factor than the total amount of radiation received. For,example, the intensity of ultraviolet radiation appears to have varied within wide limits in the past and is highest during times of high sunspot activity. A telling clue comes from observations of the planet Mars. Mars, like Earth, possesses polar caps that grow white during the Martian winter seasons and sometimes disappear during the Martian summer seasons.
During one particularly strong period of solar activity, one of the poles of Mars diminished over a period of a few days and finally disappeared. As the solar activity abated, the Martian ice cap slowly returned.
Perhaps the glacial activity on Earth, and changes in the level of the world ocean, are controlled at least in part by changes that take place beyond our planet.
Perspectives on Climate Change and Sea Level Variation
Sea level is subject to numerous short-term changes, some of considerable magnitude. The principal ones are tidal fluctuations, wind-generated waves, barometrically influenced surges, tsunamis, freshwater floods, global temperature change, and even the waves produced by passing ships. In spite of all variations, which may amount to 10 or more meters in vertical range, it is still possible to define mean sea level and to measure changes in it of the order of 1 mm y-' by using state of the art instrumentation. This quality of instrument has been used in eleven Pacific island countries under the South Pacific Sea Level and Climate Monitoring Project program to measure a long-term sea level change in the Pacific region.
For the recent past, changes in sea level relative to a particular area can be determined by analysis of tide gauge records. This is a complicated task because it has to take into account numerous seasonal and occasional events as well as regular tidal fluctuations incorporating over 100 terms arising from astronomical motions, before a reliable estimate of sea level change can be made. In fact, a partially complete cycle of long-term tidal fluctuation takes approximately 20 years. It means that the length of observed sea level data is often less than that period. Consequently, tidal influences may not be adequately removed from the investigation so that the trends contained in the observed series are significantly perturbed. In other words, a genuine sea level trend may be achieved only from long-term sea level observations.
Some sea level trends at different locations are given for comparison. The 10 year trends over 1970-1979 period are seen to be sometimes very different from the longer term trends. These results clearly indicate that the long-term sea level observation is essential to achieve a reliable sea level trend. It is also to be noted that the problem could be lack of datum control. The changes of mean sea level over periods of decades or longer are of great importance for coastal development and for the design of coastal defenses against flooding. Over periods of centuries or longer, the changes may be too great for any economically viable artificial barriers to give protection against flooding. However, it is sensible to allow in the design of coastal defense systems for predicted changes a century or so ahead.
Conclusions
Any changes in the radiative balance of the earth, including those due to an increase in greenhouse gases or in aerosols, will tend to alter atmospheric and oceanic temperature and the associated circulation and weather patterns. Generally the natural rate of the climate change throughout history has been, in human term, very slow. Evidences show that climate changes have taken many tens of thousands of years to occur. The natural rate in the past gives us an appreciation of the possible rate of climate change which is believed to be occurring because of the human activities.
Perspectives on Climate Change and Sea Level Variation
A necessary starting point for the prediction of changes in climate due to increases in greenhouse gases and aerosols is an estimate of their future concentrations. This requires a knowledge of both the strengths of their sources (natural and man-made) and also the mechanisms of their eventual removal from the atmosphere (their sinks). The projections of future concentrations can be used in climate models to estimate the climatic response. We also need to determine whether or not the predicted changes will be noticeable above the natural variations in climate. Observations are essential in order to monitor climate, to study climatic processes and to help in the development and validation of models.
The current predicted change in the earth's climate, increased through the human activities, appears to be at a rate far greater than past climatic episodes. These could be catastrophic climatic events with global warming and sea level changes of several meters happening over decades rather than millennium.
The most obvious consequences of increasing sea level would be coastal erosion and the flooding of low-lying land. When elevated high tides start to flow over the surface of islands, habitation will clearly be impossible. However, sea level rise will make low islands inhabitable long before the island itself is underwater because of the effects of sea water on soil, ground water, coastal erosion, etc.
In addition to the more obvious long term effects which are threatened by climate change and global warming, other associated hazards are possible. It is suggested that global warming may well increase the temperature and pressure gradients on the Earth's surface which in turn suggests that the incidence of severe weather effects, tropical cyclones, storm surges and like phenomena might increase significantly. Even if the world scale average incidence were to be maintained, it is highly likely that those latitudinal zones currently exposed, say, to tropical cyclones might well shift significantly. Consequently, locations not previously accustomed to such hazards may find the at such events will be a common future experience.
The question remains: what should we do now in anticipation of sea level rise? It would be premature to develop enhanced and expensive sea defenses. Global increases in atmospheric carbon dioxide are well established. There is also evidence of increased global temperature. However, there is no evidence yet of global sea level increase over and above the 10-15 centimeters per century which has been proceeding since tide gauge records began in the early nineteenth century. It is supposed that an increase in sea level should follow from an increase of air temperature due to the melting of ice and the warming and expansion of ocean water. This is a likely hypothesis based on careful but limited scientific analysis.
A first priority must be to improve the scientific understanding on which these predictions are based, in order to reduce the uncertainties. This research will involve many separate scientific disciplines, including glaciologists, oceanographers, geodesists, geologists and atmospheric scientists. The danger is that although there may be only a small rise of sea level initially, the real effects may be concealed for several decades by the thermal inertia of the ocean, meanwhile an irreversible process may be established.
Another priority is the establishment of a wide sea level monitoring system for the region of interest, with gauges measuring to common standards as part of a well distributed network. An initial thought might be that measuring the height of a liquid is a simple task. But when that liquid is disturbed by surface waves, tides, storm surges, tsunamis, and the other long-term effects involving water properties, briefly discussed in this Module, it becomes a task which broadens into being on the fringe of impossibility and requires all modern Physics and Electronics to achieve this kind of measurement. Namely, SEAFRAME [SEAlevel Fine Resolution Acoustic Measuring Equipment] has been installed. The design of the South Pacific Sea Level and Climate Monitoring Project was based upon the proposals made by, the Founding Director of the National Tidal Facility, The Flinders University of South Australia. The generous financial support has been contributed by the Australian Government through AusAID [Australian Agency for International Development]. These sea level measurements from SEAFRAME are extremely accurate and they are a long-term commitment to identifying and understanding sea level changes for future generation.
The problems of climate change and modeling and forecasting what may occur due to global warming are very severe. Despite much international research effort, still many questions are unanswered. It is a major policy of this Project that in the interest of South Pacific community under future generation, the most responsible action which we can take is to conduct a high resolution sea level measurement over the broad region of the Pacific and neighboring oceanic areas. In so doing, it is planned to accumulate a very valuable data bank of sea level data which as the years, perhaps 20 years, pass will accurately define the sea level trends under the variability, so that the present and future concerns will dissolve.
The fundamental aim of the project is to provide the Pacific Island Countries with the best available information and interpretation of the scientific consensus with regard to the climate change mechanism at a time when these communities, most at risk from sea level rise, were exposed to much media sensationalism. There was proposed an information and training program for all levels of society, from the general public, through the media, the technical communities, and the policy makers. During Phase 11 of the Project, we are focusing upon capacity building especially for climate change and sea level issues in the project member countries and these curriculum modules are major parts of this program.
